Temperature is a fundamental physical quantity, and virtually all processes in nature are closely related to it. Temperature sensors are among the earliest developed and most widely used types of sensors. Their market share far exceeds that of other sensor categories. The use of temperature for measurement dates back to the early 17th century. With the advancement of semiconductor technology, this century has witnessed the development of semiconductor thermocouple sensors, PN junction temperature sensors, and integrated temperature sensors. Correspondingly, based on the principles of wave-matter interactions, acoustic temperature sensors, infrared sensors, and microwave sensors have also been developed. When two conductors made of different materials are joined at a point and that junction is heated, an electromotive force (voltage) is generated between the unheated ends of the conductors. The magnitude of this voltage difference depends on the temperature of the unheated measurement points and the materials of the two conductors. This phenomenon can occur over a wide temperature range. By precisely measuring this voltage difference and knowing the ambient temperature at the unheated ends, the temperature at the heated junction can be accurately determined. Since this effect requires two different conducting materials, it is called a "thermocouple." Thermocouples made from different material combinations are suitable for different temperature ranges and exhibit varying sensitivities. The sensitivity of a thermocouple refers to the change in output voltage per 1°C change in the temperature at the heated junction. For most metal-based thermocouples, this value typically ranges between 5 to 40 microvolts per °C. Thermocouple sensors have their own advantages and limitations. They generally have relatively low sensitivity and are susceptible to interference from environmental signals and temperature drift in preamplifiers, making them less suitable for measuring tiny temperature changes. However, because the sensitivity of thermocouple sensors is independent of the thickness of the materials used, extremely fine materials can be employed to create temperature sensors. Additionally, the metals used in thermocouples possess excellent ductility, enabling these delicate temperature-sensing elements to achieve extremely fast response speeds, making them capable of measuring rapidly changing processes. Among the wide variety of sensors available, temperature sensors are among the most commonly used. Modern temperature sensors are designed to be extremely compact, which has further expanded their applications across various fields of industrial production and has brought countless conveniences and functionalities to our daily lives.

A ​​thermistor​​ is a ​​temperature-sensitive resistor​​ whose resistance changes significantly with temperature. Types of Thermistors: ​​By Structure/Shape​​: Spherical, rod-shaped, tubular, disc-shaped, ring-shaped, etc. ​​By Heating Mode​​: Direct-heating (self-heating) and side-heating (external heating). ​​By Working Temperature Range​​: Normal temperature, high temperature, ultra-low temperature. ​​By Temperature Coefficient​​: ​​Positive Temperature Coefficient (PTC)​​: Resistance increases with temperature (e.g., BaTiO₃-based). ​​Negative Temperature Coefficient (NTC)​​: Resistance decreases with temperature (most widely used, e.g., MnO₂-based). Key Characteristics: ​​High Sensitivity​​: Resistance changes rapidly with small temperature variations. ​​Nonlinearity​​: NTC/PTC resistance-temperature relationships are nonlinear (e.g., exponential for NTC). ​​Applications​​: Temperature measurement (e.g., thermostats), overcurrent protection (PTC fuses), temperature compensation (in circuits). Nominal Value Note: The ​​nominal resistance​​ is measured at 25°C. Actual resistance may deviate due to self-heating or material characteristics. For example, PTC thermistors show a sharp resistance increase above a critical temperature, while NTC thermistors exhibit exponential decay.

A ​​thermistor​​ (short for "thermal resistor") is a ​​temperature-sensitive semiconductor device​​ whose ​​resistance changes significantly with temperature​​. Its working principle relies on the ​​temperature-dependent electrical properties of semiconductor materials​​, primarily metal oxides like manganese, nickel, or cobalt .

(1) ​​Visual Inspection​​ First, observe the thermistor's exterior. Ensure the potentiometer or thermistor has ​​clear markings​​, with ​​no corrosion​​ on solder tabs or pins. The rotating shaft should turn ​​smoothly​​ with ​​appropriate tightness​​, and there should be ​​no mechanical noise or jitter​​ during rotation. (2) ​​Check for Loose Connections​​ Gently shake the solder tabs or pins of the potentiometer or thermistor. There should be ​​no looseness​​ detected. (3) ​​Resistance Measurement​​ Set the multimeter to the appropriate ​​resistance range​​ and perform ​​ohm zero adjustment​​. Connect the multimeter probes (ignoring polarity) to the thermistor's two terminals. Measure the actual resistance value. Compare the measured value with the thermistor's ​​nominal value​​: If the pointer ​​does not move​​, the internal resistor is ​​open-circuited​​ (damaged). A significant deviation from the nominal value indicates a fault. (4) ​​Contact Point Test​​ Connect one probe to the ​​center pin​​ (linked to the internal moving contact) and the other to any other terminal. Slowly rotate the shaft. The meter needle should move ​​smoothly and correspondingly​​. ​​Jumping or dropping​​ of the needle suggests ​​poor contact​​ between the moving contact and resistor element.